PD has a bit of a problem here, or more specifically four of them. They are called GT1, GT2, GT3 and GT4. What would happen to Daimler-Benz if the new S-Class was not much better than the C class, for S class money.
The previous GT titles have set a standard that GT5 just can't live up to and that's why they get a lot of flak and to be brutally honest, they deserve some of it. Add to that PD's annoying habit of fixing something that isn't broken and replacing it with something genuinely useless. The trend started with GT3 when the genuinely loved feature of race modification to almost all cars was replaced by, well nothing. The B-Spec mode in GT4 was a brilliant idea. If you didn't like a specific race, you could have a B-Spec driver do the job for you and you didn't have to ride 24h enduros by yourself, because you could hand over to a B-Spec-er. They saw that it makes entirely too much sense and replaced it by the mindbogglingly useless B-Spec mode of GT5. If you make such ridiculous design choices, you'll have to face the disappointment of your customer base. And if you take 5 years to come up with it, you shouldn't be surprized by airborne excrements heading towards the rotating cooling device.
With the exception of the online playing, GT5 offers vastly less than GT4 for the same or even more money. Just nice graphics don't make the cut anymore in 2010/2011 and only the die-hards really notice most of the changes to the physics model.